Two major questions in the cytochrome P450 field remain unanswered. First, are P450s required for the process of chemical carcinogenesis in an intact animal? Only studies done in vitro and in cell culture infer the importance of these enzymes. Second, do xenobiotic-metabolizing P450s also have an important, physiologically-relevant role in metabolism of endogenous hormones or other chemicals in animals? Are these enzymes required for development and survival? These questions can be answered by constructing mice that lack a functional P450 by the "gene knockout" strategy. We have focused on two P450s, CYP1A2 and CYP2E1 that are among the most well conserved in animals and that metabolically activate known chemical carcinogens. CYP1A2 is the principal P450 responsible for metabolic activation of arylamine and heterocyclic arylamine carcinogens and the most potent human hepatocarcinogen aflatoxin B1. CYP2E1 activates low molecular weight nitrosamines and a number of low molecular weight cancer suspect chemicals including benzene and vinyl halides. We are also studying receptors for foreign chemicals. These have no known function in animals and no clearly defined endogenous ligands. Therefore, the xenobiotic-receptor genes encoding the peroxisome proliferator-activated receptor, PPAR, and the Ah receptor are being subjected to the gene knockout strategy. Peroxisome proliferators are tumor promoters in rodents and also activate genes encoding P450s and peroxisomal enzymes. Many inducers that require the Ah receptor are known carcinogens and tumor promoters. Thus, the gene knockout studies can yield insight into the endogenous function and role in disease of receptors that bind foreign chemicals. These receptors are also highly conserved in mammals indicating a possible critical function. Their binding of carcinogens and tumor promoters might be required for the process of chemical carcinogenesis. Germ line transmissions of damaged alleles for CYP1A2, PPAR and the Ah receptor have been obtained. The progeny mice containing only one copy of each gene appear to be normal. The heterozygotes are currently being mated to produce homozygous mice lacking the genes.